Google has rolled out a Panda algorithm update on September 27, 2012.
This is not exactly a data refresh, in fact it’s an algo updates to the existing one. This is Google Panda 20, because this is the 20th Panda update so far.
This is a quite major update impacting 2.4% of English search queries and is still rolling out, for the next 3 to 4 days. Matt Cutts has confirmed this news to SearchEngineLand via the below mail -
Google began rolling out a new update of Panda on Thursday, 9/27. This is actually a Panda algorithm update, not just a data update. A lot of the most-visible differences went live Thursday 9/27, but the full roll out is baking into our index and that process will continue for another 3-4 days or so. This update affects about 2.4% of English queries to a degree that a regular user might notice, with a smaller impact in other languages (0.5% in French and Spanish, for example).
For your info, the last Panda update was on Sep 18 and it affected <0.7% of search queries.
CONFUSION: On the very next day, September 28, Google has also released an algorithm update for exact-match domain, which will demote the low quality exact-match domains from SERPs. And, it affected 0.6% of English-US queries.
Now, lots of webmasters are noticing huge drop in traffic. But many are not confirmed, whether it is due to exact-match domain or Panda 20 update. Because both algo updates were released almost simultaneously (1 day difference).
P.S. - Even though you have an exact-match domain name, if you have thin content or low quality content in your website, penalty by Google Panda is most likely.